FUNCTION-DEPENDENT ANATOMICAL PARAMETERS OF RABBIT MASSETER MOTOR UNITS

Rabbit masseter motor units (22) were studied by stimulation of trigem inal motoneurons. We tested the hypotheses that masseter motor units f acilitate fine motor control by concentrating fibers in small areas an d that the distribution of motor unit fibers depends on the fiber type . The twitch contraction time and the isometric tetanic force were reg istered. The motor unit fibers were depleted of their glycogen by prol onged stimulation. Serial sections of the entire muscle were stained w ith the periodic acid Schiff (PAS) and monoclonal antibody stains. The muscle fibers of the motor unit were mapped and identified by four my osin heavy-chain (MHC) isoforms: I, IIA, IID, and cardiac-alpha. In th e PAS-stained sections, anatomical parameters of the motor units, affe cting the force output, were analyzed: the innervation ratio (IR), mot or unit territory area (TA), and relative (R-DENS) and absolute (A-DEN S) motor unit fiber densities. The fiber cross-sectional area (F-CSA) was measured for each MHC fiber type. The F-CSA sum of all motor unit fibers, the physiological cross-sectional area (P-CSA), was calculated . The IR ranged between 77 and 720 fibers (mean, 267). The mean TA was 8.71 mm(2) (range, 4.45 to 19.58). The mean R-DENS was 10 fibers per 100; the A-DENS was 31 fibers per mm(2). Linear correlations were foun d between the IR and the R-DENS and between the tetanic force and the IR. The F-CSAs showed a stepwise increase in value from type I- to IID -MHC fibers. The mean P-CSA was 0.90 mm(2) (range, 0.09 to 2.97). A hi gh linear correlation was noted between the P-CSA and the tetanic forc e. Ln conclusion, increase of motor unit size expressed in higher fibe r counts and forces is accomplished by increase of the fiber density. Thus, forces can be exerted selectively in restricted regions of the m asseter muscle. Differences in fiber orientation due to complex muscle pinnation emphasize the possibility of an accurate muscle performance .